The present invention relates to an impregnated cathode used for an electron tube and a method for manufacturing the same.
An impregnated cathode has a basic structure in which pores of a sintered body of porous metal (pellet) are impregnated with an electron emitting material. A method for manufacturing an impregnated cathode comprises the steps of: press molding powder of a high melting point metal such as tungsten, etc.; then sintering the press molded product to form a reducing substrate having a proper porosity; and then impregnating the pores of the substrate with molten electron emitting material comprising BaO, CaO and Al2O3 as the main components. Thus, a cathode pellet is obtained. This cathode pellet is impregnated with emitting material in an amount corresponding to the volume of the sintered body and the porosity, i.e. the volume of pores.
The principle of operation of the cathode pellet will be explained below. When the cathode pellet is subjected to a high temperature activation, BaO is reduced by the pellet to generate free Ba. This free Ba thermally diffuses in pores and reaches the surface of the pellet. Then, the free Ba thermally diffuses on the surface of the pellet, to thus form a Ba monoatomic layer on the surface of the pellet. At this time, a monoatomic layer spreads to cover an area corresponding to the difference between an amount of Ba evaporated from the monolayer, which is dependent upon the temperature of the pellet, and an amount of Ba supplied from the inside of the pellet. This Ba monoatomic layer reduces the effective work function that is involved in an electron emission from 4 to 5 eV of the metal itself constituting the pellet to about 2 eV. Consequently, excellent thermionic emission is provided.
If little Ba is supplied from the inside of the pellet at the time of the operation, a necessary and sufficient area of Ba monoatomic layer cannot be formed, causing a deficiency of emission. Moreover, there arise some problems, for example, the activation takes a long time, etc.
On the contrary, if too much Ba is supplied, Ba evaporated from the surface of the pellet is increased, so that the BaO impregnated in the pellet is consumed in a short time and in turn the lifetime is shortened. Furthermore, the evaporated Ba is deposited on a counter electrode, causing unnecessary electron emission, etc.
The most important point of the operation of the impregnated cathode is to form a necessary and sufficient Ba monoatomic layer in an early stage and to keep it for a long time. The factors for forming a Ba monoatomic layer include: the amount of impregnated BaO; the reducing rate of the impregnated BaO being reduced by the pellet; the thermal diffusion velocity of free Ba in pores; and the surface thermal diffusion rate of Ba on an electron emitting face.
The design parameters for controlling the operations are: the amount of impregnation of electron emitting material; the porosity of the pellet and the spatial distribution of pores; and the cleanness of the electron emitting face, more specifically, an absence of extra electron emitting material attached to the electron emitting face. The most important thing for mass production is to control these parameters with high precision and with less variation.
Based on the above mentioned background of the principle, Publication of Japanese Patent Application (Tokko Sho) No. 44-10810 discloses an impregnated cathode, in which the evaporation of extra electron emitting material can be inhibited, the leak of current in an insulating portion of an electron gun can be reduced, and an excellent state of Ba monoatomic layer can be maintained for a long time and in turn its lifetime can be extended.
The above mentioned structure is a two-layer structure comprising a first layer having a low porosity on the side of the electron emitting face of the pellet, wherein the evaporation of the electron emitting material is inhibited; and a second layer having a high porosity formed below the first layer. According to such a two-layer structure, even after the Ba supply capacity of the first layer is exhausted (i.e. after the lifetime), Ba can be supplied from the second layer to the first layer. Consequently, the lifetime of the pellet is further extended as compared with the lifetime the first layer has naturally.
Furthermore, Publication of Japanese Patent Application (Tokkai Hei) No. 6-103885 suggests that the surface roughness of the substrate be not more than 5 xcexcm, more preferably that the substrate be perfectly smooth, so as to easily remove the attached extra electron emitting material after impregnation.
Furthermore, Publication of Japanese Patent Application (Tokkai Sho) No. 58-87735 discloses a manufacturing method in which compressed electron emitting materials placed on the upper surfaces of the individual pellets are melted and impregnated in order to ensure the amount of impregnation of the electron emitting material.
Furthermore, Publication of Japanese Patent Application (Tokkai Hei) No. 6-103885 discloses a method of mass production in which the amount of the impregnated electron emitting materials is kept stable by classifying metal raw material powder of the pellet and controlling the porosity of the pellet.
Furthermore, a mechanical method using a brush, a metal-clad needle, etc., a polishing method by means of cutting, etc., and ultrasonic cleaning in water, etc. have been conventionally suggested.
Furthermore, Publication of Japanese Patent Application (Tokkai Sho) No. 50-103967 discloses a method in which a pellet is provided on the specific jigs one by one and then washed by ultrasonic cleaning in clean water.
However, the above mentioned conventional impregnated cathodes have the following problems.
(1) In order to manufacture the impregnated cathode having a two-layer structure, it is necessary to use two different kinds of raw material powders or to carry out press molding twice. Consequently, the production process is complicated.
(2) In the method in which a pellet is treated one by one or the raw material powder is classified, the productivity is poor and mass production is difficult.
(3) The method of mechanically removing extra electron emitting materials by using a brush, metallic needle, etc., is difficult to carry out. Furthermore, a treatment is necessary for each pellet, so that mass production is difficult.
(4) The manufacturing process in which the sintered pellets are provided on the specific jig one by one is complicated. It takes not less than 1 hour to perfectly remove extra electron emitting materials by way of only the ultrasonic cleaning method. Consequently mass production is difficult.
It is the object of the present invention to solve the above mentioned conventional problems and to provide an impregnated cathode and a method of manufacturing the same, which is excellent in initial electron emitting performance, lifetime property, and insulating property and which is suitable for mass production by continuously increasing the porosity of the sintered body of porous metal as the distance in the depth direction from the electron emitting face is increased.
In order to achieve the above mentioned objects, the first impregnated cathode of the present invention has a cathode pellet in which the pore portion of a sintered body of porous metal is impregnated with electron emitting material, wherein the porosity of the sintered body of porous metal is continuously increased as the distance in the depth direction from an electron emitting face is increased.
By the above mentioned impregnated cathode, since no discontinuity of the porosity in the pellet is formed, a reaction generating free Ba proceeds continuously and smoothly all over the pellet. Moreover, since raw material powder having more than one kind of particle sizes need not be used, the manufacturing process can be simplified.
It is preferable in the above mentioned first impregnated cathode that the porosity of an electron emitting face of the sintered body of porous metal is in the range of 12.5 to 25 volume %; the porosity difference between the porosity of a vicinity of the electron emitting face and the porosity of a vicinity of the opposite face to the electron emitting face is in the range of 5 to 25 volume %; and the porosity of the opposite side to the electron emitting face is less than 40 volume %. With such an impregnated cathode, an excellent lifetime property can be obtained.
It is further preferable in the first impregnated cathode that the surface roughness of the electron emitting face of the cathode pellet is in the range of 5 to 20 xcexcm for the maximum height. With the above mentioned impregnated cathode, the emission property can be enhanced.
Next, according to a first method for manufacturing an impregnated cathode of the present invention, a method for manufacturing an impregnated cathode having a cathode pellet in which the pore portion of a sintered body of porous metal is impregnated with electron emitting material, comprises the steps of press molding metal raw material powder to form a porous substrate, the press molding being conducted after filling the metal raw material powder in a struck-level cartridge and then filling the raw material metal powder in a die by level striking measurement; wherein a contacting face of the cartridge and the die surface has an annular shape and the cartridge has an inclined face in which the end portion of the outside of the cartridge contacts with the die surface.
According to the above mentioned manufacturing method, the level striking measurement can be conducted exactly, so that the particle size distribution of the raw material powder inside the cartridge can be reflected in the particle size distribution of the raw material to be filled in the press die. Consequently, the variation of the porosity of the pellet or manufacturing variation in the amount of impregnation of electron emitting materials can be reduced.
It is preferable in the first method for manufacturing an impregnated cathode that the inner diameter of the annular shape is in the range of 10 to 20 times as large as the diameter of a pellet; the external diameter of the annular shape is in the range of 1.05 to 1.3 times as large as the inner diameter; and the angle that the inclined face makes with the die face is in the range of 40 to 80xc2x0.
It is further preferable that an amount of metal raw material powder that is filled in the cartridge is equal to an amount of 200 to 800 cathode pellets.
It is further preferable that the metal raw material powder is heated at temperatures in the range of 50 to 100xc2x0 C. at the time of level striking measurement and pressing.
It is further preferable that a face at which a punch contacts with metal raw material powder is referred to an electron emitting face; the relative descending speed of the punch to the die is in the range of 0.5 to 5 cm/s, and the pressing time is in the range of 1 to 7 seconds when the punch contacts with metal raw material powder.
Next, according to the second method for manufacturing an impregnated cathode of the present invention, a method for manufacturing an impregnated cathode having a cathode pellet in which the pore portion of a sintered body of porous metal is impregnated with electron emitting material comprises the steps of: press molding metal raw material powder to form a porous substrate; and sintering the porous substrate to form a sintered body of porous metal; wherein the average porosity of the porous substrate after press molding is controlled by adjusting the pressure of press molding, and the average porosity of the sintered body of porous metal after sintering is controlled by adjusting the sintering temperature.
By the above mentioned method for manufacturing the impregnated cathode, it is not necessary to use raw material powder having a different particle sizes and to mold in multilayers. Consequently, the average porosity of the entire pellet can be controlled by the general process.
It is preferable in the second method for manufacturing an impregnated cathode that porosity distribution is controlled by adjusting the descending speed of the punch and the pressing time. By the above mentioned method for manufacturing an impregnated cathode, it is not necessary to use raw material powder having different particle sizes and to mold in multilayers. Consequently, the average porosity of the entire pellet can be controlled by general process.
Furthermore, it is preferable that an average porosity (D volume %) of the porous substrate after press molding and an average porosity (d volume %) of the sintered body of porous metal after sintering has a relationship expressed by the following equation:
d+10xe2x89xa6Dxe2x89xa6d+20.
By the above mentioned method for manufacturing an impregnated cathode, the pellets that ensures a certain amount of impregnation can be manufactured by maintaining the mechanical strength and inhibiting the generation of closed pores.
Next, according to the third method for manufacturing an impregnated cathode of the present invention, a method for manufacturing an impregnated cathode having a cathode pellet in which a pore portion of a sintered body of porous metal is impregnated with electron emitting material comprises the steps of placing the sintered body of porous metal and the electron emitting material in a container for impregnation in such a manner that the electron emitting material contacts with an entire surface of the sintered body of porous metal when the electron emitting materials are melted, and impregnating the pore portion of the sintered body of porous metal with the electron emitting material.
With the above mentioned impregnated cathode, deficiency of impregnation can be prevented. Consequently, stable impregnation can be obtained.
It is preferable in the third method for manufacturing an impregnated cathode that electron emitting materials are filled in the container for impregnation in such a manner that the depth of the electron emitting materials is uniform, and the sintered body of porous metal is located at the middle portion in the direction of the depth of the electron emitting material or located at the top of the electron emitting material.
It is further preferable in the third method that the weight of the electron emitting material to be filled in the container for impregnation is in the range of 10 to 100 times as much as the impregnatable weight of the sintered body of porous metal in the container for impregnation. Herein, impregnatable weight means the total effective weight of emitting material that is carried by the porous sintered bodies, or something similar. By the above mentioned method for manufacturing an impregnated cathode, the variation of the amount of impregnation can be reduced.
It is further preferable in the third method that extra electron emitting materials are removed by shaking a container in which an impregnated cathode pellet and alumina ball are placed and washing by ultrasonic cleaning in water. By the above mentioned method for manufacturing an impregnated cathode, extra electron emitting materials can be removed while inhibiting the fracture rate of the pellet and the variation of the amount of impregnation can be reduced.